With reference to FIG. 1 for an improved disposable safety syringe structure (as disclosed in R.O.C. Utility Model No. M360049), the improved disposable safety syringe structure comprises a cylindrical body 11, a needle seat 12, and a plunger 3, wherein the plunger 3 has a piston head 131, and a latch edge 133, and the cylindrical body 11 has a flange 16 and two positioning bases 111, and each of the positioning bases 111 has an engagement slot 112, and the needle seat 12 has a fitting section 121 disposed at a front end of the needle seat 12 for combining an injection needle 14 and an elastic inverted hook plate 122 disposed at a rear end of the needle seat 12, and the needle seat 12 has a plurality of convex column engagement members 123 disposed at a front end of the needle seat 12. When the needle seat 12 is assembled into the cylindrical body 11, the convex column engagement members 123 are engaged with the engagement slots 112 of the positioning bases 111 of the cylindrical body 11 respectively. The piston head 131 has a piston plate 15, and the piston head 131 has a circular flange 132 disposed at a front end of the piston head 131. After medicine is injected, the piston head 131 can be forced to plug into the needle seat 12, and the circular flange 132 of the piston head 131 is hooked to the inverted hook plate 122 of the needle seat 12, so that the piston head 131 and the needle seat 12 are coupled integrally as a whole, and the piston head 131 drives the convex column engagement member 123 of the needle seat 12 to be separated from the engagement slot 112 of the positioning base 111 of the cylindrical body 11, and the piston head 131 of the plunger 3 is retracted to drive the needle seat 12 and the injection needle 14 to be retracted into the cylindrical body 11 to achieve the effects of hiding the needle seat 12 and the injection needle 14 and preventing injuries or accidents.
Although the foregoing improved disposable safety syringe structure can achieve the effect of preventing accidents or injuries effectively, the disposable safety syringe structure just can prevent the plunger 3 from detaching from the cylindrical body 11 by interfering the piston plate 15, the latch edge 133 and the flange 16. However, if the design of the relation between the latch edge 133 and the flange 16 is too loose, then when the injection needle 14 extends into the cylindrical body 11, the injection needle 14 may be retracted to the outside and may cause accidents. If the design of the relation between the latch edge 133 and the flange 16 is too tightly, then when the plunger 3 is installed into the cylindrical body 11 and pushed forward, the plunger will not be pushed easily. Therefore, the conventional structure of the latch edge 133 and the flange 16 requires further improvements.